1. Field of the Invention
The present invention relates to a power supply system for use in a hybrid vehicle in which is mounted an engine for power generation as well as a drive motor for vehicle propulsion, and more particularly, is directed to a drive control of a starting motor for actuating the power generation engine.
2. Description of the Related Arts
A hybrid vehicle taking advantage of both the lower polluting properties peculiar to an electric vehicle and the continuous traveling performance associated with an engine powered vehicle has been hitherto proposed. This type of hybrid vehicle usually travels as an electric vehicle through the drive of a motor by electric power derived from a main battery. Furthermore, the state of charge (hereinafter, abbreviated to SOC) of the battery is monitored during traveling. When the SOC becomes less than a predetermined lower level, the engine is actuated and the vehicle travels while performing power generation. Thus, the main battery is charged with electric current produced by a power generator. When the SOC is restored to a predetermined higher level, the engine is stopped and the hybrid vehicle is allowed to travel as an ordinary electric vehicle.
In this manner, the hybrid vehicle also includes an engine which is driven exclusively for the purpose of charging the battery and hence is operated under a fixed load and a fixed number of revolutions. Therefore, unlike the engine vehicle using the engine as its drive source, there can be realized decreased noise generation and much lower contaminant content in the exhaust gas (Refer to Japanese Patent Laid-open Publication No. 56-166704, for example).
The hybrid vehicle is commonly equipped with an auxiliary battery which provides electric power to an electronic control unit (hereinafter abbreviated as ECU) which carries out a variety of control functions such as control of motor driving conditions and engine starting control, and to accessories such as headlights. The auxiliary battery is adapted to receive electric power from the main battery with the aid of a DC-DC converter.
This auxiliary battery is intended to supply electric power to a starting motor for starting the engine as well. For the drive of the starting motor, the auxiliary battery is required to consume the largest amount of electric power. As described above, the actuation of the engine is carried out depending on the SOC of the main battery. It is therefore ordinarily impossible to control the timing of driving the motor. As a result, the voltage at the auxiliary battery may be disadvantageously substantially lowered when it has been already lowered due to turning on headlights and windshield wipers at night or under rainy weather, or when the starting motor is continuously driven due to failing to actuate the engine.
Hence, there arose a problem that the ECU malfunctions in the case where the voltage at the auxiliary battery is substantially lowered, which results in a poor supply of electric power into the motor.